A great diversity of oligosaccharide structures and types of glycoconjugates is found in nature, and these are synthesized by a large number of glycosyltransferases. Glycosyltransferases catalyze the synthesis of glycoconjugates, including glycolipids, glycoproteins, and polysaccharides, by transferring an activated mono- or oligosaccharide residue to an existing acceptor molecule for the initiation or elongation of the carbohydrate chain. A catalytic reaction is believed to involve the recognition of both the donor and acceptor by suitable domains, as well as the catalytic site of the enzyme (Amado et al. (1999) Biochim Biophys Acta 1473:35-53; Kapitonov et al. (1999) Glycobiology 9:961-78).
Because the glycosylation reaction is highly specific with respect to both the configuration of the sugar residue and the site of the addition, it is expected that unique domain structures for substrate recognition and nucleotide-sugar binding are located within the enzyme molecule. Evidence indicates that formation of many glycosidic linkages is covered by large homologous glycosyltransferase gene families, and that the existence of multiple enzyme isoforms provides a degree of redundancy as well as a higher level of regulation of the glycoforms synthesized (Kapitonov et al. (1999) Glycobiology 9:961-78).
Glycosylation is the principal chemical modification to proteins as they pass through Golgi vesicles. Glycosyltransferases of the Golgi do not possess an obvious sequence homology which would suggest a common Golgi retention signal. However, they are all membrane proteins and share type II topology, consisting of an amino terminal cytoplasmic tail, a signal anchor transmembrane domain, a stem region, and a large luminal catalyitc domain. The membrane-spanning domain and its flanking regions contain necessary and sufficient information for Golgi retention of these enzymes (Jaskiewicz (1997) Acta Biochim Pol 44:173-9). ER localized glycosyltransferases can have either a type II topology, like the Golgi glycosyltransferases, or a type I topolgy, e.g., the N-terminus and catalytic domain inside the ER (Kapitonov et al. (1999) Glycobiology 9:961-78). Some glycosyltransferases are present on the cell surface and are thought to function as cell adhesion molecules by binding oligosaccharide substrates on adjacent cell surfaces or in the extracellular matrix. The best studied of these is beta 1,4-galactosyltransferase, which mediates sperm binding to the egg coat and selected cell interactions with the basal lamina (Shur (1993) Curr Opin Cell Biol 5:854-63).